delericho
Legend
Have you never actually RUN a mystery adventure?
Actually, as it happens I'm running an entire mystery campaign right now!
The worst thing in the world would be to prepare a complex intricate mystery, and have the players miss the first few clues; half an hour in and they're standing around going "What the hell are we suppose to do? THIS SUCKS."
If there is a mystery they must solve in order to resolve the adventure then YES, you guarantee, somehow, that the players WILL solve it. If the secret door leads to something they MUST do, or have, or know, then you guarantee that they WILL find it. If the players NEED to figure it out in order to move forward, than failure to figure it out means the game is over, just as surely as if you TPKd them.
All of this is true. If the players must find the 'secret' door, or must find a specific clue, then the DM must, at the end of the day, find a way to give it to them.
But that's a bad way to design a mystery game. You need to build in multiple layers of redundancy, because PCs will miss some (or many) of your clues. You probably even need multiple mysteries going on, so if they get stuck on one front they can still progress on others. And so on.
(Also, you want at least some clues that aren't found by rolling the dice - if the players think to speak to NPC X, they automatically get a clue; the victim's home has some clues out in plain sight, whatever.)
But, in any case, I wasn't talking about a mystery adventure, since that's a fairly unusual type of adventure with special design considerations. I was instead addressing a more 'typical' D&D scenario. In such a scenario there generally shouldn't be any secret door that the party must find, or a riddle that they must solve. And, moreover, I take the view that both the DMs and the designers must adopt the view that it's okay if some of these mysteries go unfound in play. (Because if it's tied to the dice, the rolls may suck. If it's tied to player skill, they may not ask the right questions. Either way, if something is hidden, it may well never be found.)